This invention relates to a vacuum switch which is required to have a low chopping current characteristic, and more particularly to an electrode composition for such a vacuum switch composed of an alloy including copper (Cu) and a low melting point metal such as bismuth (Bi), lead (Pb), indium (In) or the like.
Conventional electrode compositions of the type referred to have involved cupper-bismuth (Cu-Bi) alloys, copper-lead (Cu-Pb) alloys, copper-cobalt-bismuth (Cu-Co-Bi) alloys, cupper-chromium-bismuth (Cu-Cr-Bi) alloys etc. When the low chopping current characteristic is not required, emphasis is placed on properties other than the low chopping current characteristic, by controlling the content of a low melting point metal such as bismuth or the like to about 1% by weight. On the other hand, where the low chopping current characteristic is required to be not higher than one ampere, the particular electrode composition includes a low melting point metal such as bismuth or the like in a large amount on the order of from 10 to 20% by weight. At that time, one or more of cobalt (Co), chromium (Cr), nickel (Ni), titanium (Ti), tungsten (W), iron (Fe) etc. has or have been added to the electrode composition for the purpose of improving the withstanding voltage characteristic. However the low melting point metal such as bismuth, lead, indium or the like scarcely forms a solid solution with copper at room temperature and is precipitated into a metallographic structure having a low melting point metal aggregated at the grain boundary of copper. This has resulted in disadvantages such that, upon interrupting a high current, a vapor of the low melting point metal is evolved in a large amount to sharply reduce the interrupting characteristic while the low melting point metal precipitated at the copper grain boundary greatly deteriorates the mechanical strength of the alloy.
Also upon brazing the electrode alloy to an associated electrode rod at a temperature of from 700.degree. to 800.degree. C., the low melting point metal intrudes in the junction of the alloy and the rod to greatly decrease the strength of the junction. Also when the electrode alloy brazed to the electrode rod is assembled into an envelope followed by the degasing and evacuating of the envelope at from 400.degree. to 600.degree. C., the low melting point metal is vaporized and scattered to contaminate the inner surface of the envelope. This has resulted in the disadvantage that the withstanding voltage characteristic is reduced and so on.
Further more each time the resulting vacuum switch is operated to open or close a load current flowing therethrough, the surface of the contact formed of the electrode alloy becomes slowly enriched with copper attended with the fatal disadvantage that the chopping current of the switch rises.
Accordingly it is an object of the present invention to provide a new and improved electrode composition for a vacuum switch improved in interrupting characteristic, withstanding voltage characteristic and/or brazing characteristic while maintaining the chopping characteristic stable and low with an indefinite number of the switching operations performed by a vacuum switch including a pair of contacts formed of such an electrode composition.